Field of the Invention
The present invention relates to an scanning probe microscope. The present invention further relates to a method of operating an scanning probe microscope.
Related Art
Scanning probe microscopes (SPM), such as Atomic Force Microscopes (AFM) are widely used for the physical characterization of materials and devices when high spatial resolution and small feature sizes are of interest. AFMs are primarily used in imaging modes to provide topographic information, but they can also record the force between a probe tip and a sample. In AFM, the probe tip is typically positioned on a cantilever spring, which serves as a flexible carrier that translates force in bending or displacement. Other flexible carriers may also be used for this purpose instead of a cantilever spring, such as diaphragms or double clamped beams. Movement of the flexible carrier is typically measured using an optical beam deflection setup, although different read-out schemes also exist. In the following description the wording ‘cantilever’ is used as a particular example of a flexible carrier. The disclosure is however equally applicable to embodiments using another type of flexible carrier, as suggested above.
In the common imaging mode ‘Amplitude Modulation’ AFM (also called ‘tapping mode’, ‘AC mode’ or ‘intermittent contact mode’), the cantilever is oscillated at or near its resonance frequency. The amplitude of this oscillation is reduced by the force between tip and surface and is measured and used as input for a feedback system which adjusts the height of the tip such that the amplitude remains constant. When the tip is scanned over the surface, the topographical variations between positions continuously change the amplitude which the feedback then corrects for. The speed of correction is limited by a number of factors, including the cantilever response time and the speed at which the cantilever can be mechanically repositioned.
Cantilever response time is determined by the cantilever resonance frequency and damping or Q factor. In this connection people have worked both at increasing the resonance frequency and decreasing the Q factor to speed up AFM imaging. The first mention in literature of reducing Q for enhanced speed is Mertz et al. in “Regulation of a microcantilever response by force feedback”, Applied Physics Letters, 1993, 62, 2344, which introduces the idea to feed back the measured cantilever motion in an external drive signal to reduce Q. Other schemes have been developed to reduce Q too, for example, Fairbairn et al. describes the use of an active impedance in a piezoelectric shunt control framework in “Sensorless Implementation of a PPF Controller for Active Q Control of an AFM Microcantilever”, IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY. Fairbairn compares results achieved with a quality factor Q of 226 and with a reduced cantilever Q factor of 17 and concludes that reducing the cantilever Q factor with active piezoelectric shunt control allows for improved image quality at high scan speeds.
It is noted that US20070024295 discloses a probe for an atomic force microscope is adapted such that, as a sample is scanned, it experiences a biasing force urging the probe towards the sample. This improves probe tracking of the sample surface and faster scans are possible. This is achieved by either including a biasing element which is responsive to an externally applied force, on the probe and/or reducing the quality factor of a supporting beam. The quality factor may be reduced by coating the beam with a mechanical-energy dissipating material. It is a disadvantage of this known probe that the coating causes a relatively substantial mechanical stress and therewith a distortion of the beam. Moreover, this mechanical stress and resulting distortion depend on external circumstances, such as temperature, which render the probe less accurate. Also non-uniformities and defects in the coating can easily result in deviations of the mechanical properties of the probe.
It is an object of the present invention to provide for an improved image quality at high scan speeds or to enable an increase of the scan speed while maintaining image quality by reducing the Q factor of the probe, while avoiding that this results in deviations of its other mechanical properties.